The present invention relates to improvement of an electric power steering apparatus for steering assistance based on an output of a torque sensor which detects a steering torque. More particularly, the invention relates to detection of malfunction for the torque sensor.
FIG. 1 is a block diagram showing the constitution of an essential part of the conventional electric power steering apparatus. In the electric power steering apparatus, a steering torque signal from a torque sensor 2 provided on a steering shaft (not illustrated) is phase compensated by a phase compensation unit 11 and given to an assist control unit 12.
Besides, a vehicle speed signal from a vehicle speed sensor 7 is given to the assist control unit 12. The assist control unit 12 outputs a target current value for assist control (steering assist control) based on the steering torque signal from the phase compensation unit 11 and the vehicle speed signal from the vehicle speed sensor 7 and gives the value to subtracting means 15.
The subtracting means 15 obtains a deviation between the target current value from the assist control unit 12 and a feed back value of the drive current of the motor 5 detected by the motor current detection circuit 6, and gives this deviation to the motor drive circuit 16. The motor drive circuit 16 determines a motor current based on the deviation and gives it to the motor 5 to drive it in rotation.
According to the conventional electric power steering apparatus as described above, when an excessively large input is applied to the torque sensor 2, the neutral point in order to the measurement may shift. However, when the displacement by the shift is relatively small, there is a problem that the displacement cannot be detected and the apparatus is doing nothing about the condition that the torque sensor 2 has large measuring errors.
As a prior art on the similar problem, there is disclosed in Japanese Patent Publication Unexamined No. H1-178080(1989) "A Fail-safe Device for An Electric Power Steering Apparatus", wherein the number of times that the input torque is reached into a certain range is counted independently for the right and left side direction, and when the difference of the number between the right and left counts exceeds a certain level, it is judged that a malfunction has occurred. With the apparatus, the malfunction can be detected only when the displacement is large enough.